Semiconductor laser

ABSTRACT

A semiconductor laser includes a first conductivity type lower cladding layer on a first conductivity type semiconductor substrate; an active layer for laser oscillation on the lower cladding layer; a second conductivity type first upper cladding layer on the active layer, the second conductivity type being opposite the first conductivity type; current blocking layers at both sides of the active layer to confine current flow to the active layer; a second conductivity type second upper cladding layer disposed on the first upper cladding layer and the current blocking layers, interfaces between the current blocking layers and (i) the lower cladding layer, (ii) the first upper cladding layer, and (iii) the second upper cladding layer being crystalline regrowth interfaces; and doped layers including at least one of Fe, Cr, and Co atoms as a dopant disposed at the crystalline regrowth interfaces. At least one of Fe, Cr, and Co enters into the crystal lattices and changes of the crystal lattices. Movements of impurities are suppressed at the crystalline regrowth interfaces, thereby suppressing laser deterioration.

FIELD OF THE INVENTION

The present invention relates to a semiconductor laser and, moreparticularly, to enhancement of reliability of a semiconductor laser.

BACKGROUND OF THE INVENTION

FIG. 6 is a cross-sectional view illustrating a prior art buriedheterostructure (BH) laser diode which is illustrated in, for example,SPIE-The International Society for Optical Engineering, Volume 2148,pages 142-151.

In the figure, reference numeral 1 designates a semiconductor substratecomprising p type InP. Reference numeral 3 designates a p type InPcladding layer including a buffer layer 2. An active layer 4 comprisingInGaAsP is disposed on the p type InP cladding layer 3. A first n typecladding layer 5 comprising n type InP is disposed on the active layer4. P type current blocking layers comprising p type InP are located onboth sides of a mesa comprising the lower cladding layer 3, the activelayer 4, and the upper cladding layer 5. An n type current blockinglayer 7 comprises n type InP. A second n type cladding layer 8comprising n type InP is in contact with the upper surface of the p typecurrent blocking layer 6. A contact layer 9 comprising N type InP formaking an ohmic contact is disposed on the second n type cladding layer8. Reference numerals 11, 12, 13, and 14 designate crystalline regrowthinterfaces, and reference numeral 10 designates an electrode comprisinga metal.

In the figure, each arrow represents a current. The crystalline regrowthinterfaces 11, 12, 13, and 14 that are formed during crystalline growthsteps, regions A, B, C, and D, represented by circles, are regions thatdeteriorate during high temperature operation and through which leakagecurrents represented by arrows flow without flowing through the activelayer 4. Deterioration is likely to occur at the pn junction interfacesbetween p type InP and n type InP, i.e., at the crystalline regrowthinterfaces 11, 12, 13, and 14, during successive crystalline growthsteps. Generally, the crystalline regrowth interfaces 11, 12, 13, and 14are likely to be exposed to air or etched, resulting in the formation ofnatural oxide films or the presence of impurities, producing manycrystalline defects.

FIGS. 7(a)-7(d) are cross sectional views for explaining the process offabricating the BH laser. As shown in FIG. 7(a), a p type cladding layer3 comprising p type InP including a buffer layer 2, an active layer 4comprising InGaAsP, and a first n type cladding layer 5 comprising ntype InP are successively grown by MOCVD (metal organic chemical vapordeposition). Then a silicon dioxide film is grown, and the silicondioxide film is patterned by photolithography to form a mask 15.

Subsequently, a shaped stripe mesa 16 is formed by etching, as shown inFIG. 7(b). The side wall of the stripe structure 16 is exposed to theetching material as well as to the air, whereby impurities and a naturaloxide film are present at the side wall of the stripe structure 16.

Next, as shown in FIG. 7(c), a p type current blocking layer 6comprising p type InP, an n type current blocking layer 7 comprisingInP, and a p type current blocking layer 6 comprising p type InP aresuccessively grown by MOCVD, thereby burying the active layer 4.Thereafter, the mask 15 is removed by etching. Then, on the surface ofthe p type current blocking layer 6 etching material remains as residualimpurities along with a natural oxide film.

Subsequently, as shown in FIG. 7(d), a second n type cladding layer 8comprising n type InP and a contact layer 9 comprising n type InP aregrown by MOCVD and, thereafter, an electrode 10 is formed.

As is apparent from this fabricating process, there are two crystallinegrowth processes and there are impurities and natural oxide films at theregions A, B, C, and D in FIG. 6, i.e., at the pn junction interfaces ofthe crystalline regrowth interfaces 11, 12, 13, and 14 between the ptype current blocking layer 6 and the first n type cladding layer 5, andbetween the p type current blocking layer 6 and the second n typecladding layer 8, whereby leakage currents that are ineffective inproducing laser oscillation flow in the regions A, B, C, and D, therebyincreasing operation current. This results in deterioration in the lasercharacteristics and deteriorated reliability.

While the foregoing description is concerned with a BH laser structureemploying a semiconductor substrate comprising p type InP, in any of theBC (buried crescent) laser structure of FIG. 8 (described in ExtendedAbstracts of the 15th Conference on Solid State Devices and Materials,Tokyo, 1983, pp.337-340), the BH laser structure employing asemiconductor substrate comprising n type inP shown in FIG. 9, the BR(buried ridge)laser structure employing a semiconductor substratecomprising n type GaAs shown in FIG. 10, there arises laserdeterioration at the regrown pn junction interfaces.

The BC laser structure shown in FIG. 8 includes a semiconductorsubstrate 1 comprising p type InP, an n type current blocking layer 18comprising n type InP, a p type cladding layer 19 comprising p type InP,an active layer comprising InGaAsP, a first n type cladding layer 21comprising n type InP, a second n type cladding layer 22 comprising ntype InP, a contact layer 23 comprising n type InP, and an electrode 10.This BC laser is fabricated by successively growing on a p type InPsemiconductor substrate 1 an n type current blocking layer 17 and a ptype current blocking layer 18, successively, forming a stripe shapedgroove on the p type current blocking layer 18 having a depth reachingthe p type InP semiconductor substrate 1 from the surface of the p typecurrent blocking layer 18, removing the mask, forming a p type InPcladding layer 19, an InGaAsP active layer 20, a first n type claddinglayer 21 by liquid phase epitaxy so as to fill the groove, andsuccessively forming a second n type cladding layer 22 and an n type InPcontact layer 23, and an electrode 10. During these processes, there areformed the crystalline regrowth interface 24 between the n type currentblocking layer 17 and the p type cladding layer 19, and the crystallineregrowth interface 25 between the p type current blocking layer 18 andthe first n type cladding layer 21 and second n type cladding layer 22,at which the laser deteriorations would occur.

The BH laser structure shown in FIG. 9 includes an n type InPsemiconductor substrate 26, an n type cladding layer 28 comprising ntype InP including a buffer layer 27, an active layer 4 comprisingInGaAsP, a first p type cladding layer 29 comprising p type InP, a ptype current blocking layer 30 comprising p type InP, an n type currentblocking layer 31 comprising n type InP, and a second p type claddinglayer 32 comprising p type InP and a contact layer 33 comprising p typeInp. This BH laser is fabricated by growing on an n type InPsemiconductor substrate 26 an n type cladding layer 28 including an ntype InP buffer layer 27, an InGaAsP active layer 4, a first p type InPcladding layer 29 successively by MOCVD, then forming a mesa shapedstripe structure by etching from the surface of p type InP claddinglayer 29 to reach the semiconductor substrate 26, employing a stripeshaped insulating film mask (not shown), regrowing the p type InPcurrent blocking layer 30 and the n type InP current blocking layer 31by MOCVD so as to bury the mesa shaped stripe, employing the mask as aselective growth mask, then removing the mask, and regrowing the secondp type InP cladding layer 32 and p type InP contact layer 33. Duringthese processes, laser deterioration occurs at the pn junctions at thecrystalline regrowth interface 34 between the n type cladding layer 28and the p type current blocking layer 30 and at the crystalline regrowthinterface 35 between the n type current blocking layer 31 and the p typecladding layer 29 and second p type cladding layer 32.

In addition, the BR laser structure shown in FIG. 10 includes asemiconductor substrate 36 comprising n type GaAs, an n type claddinglayer 37 comprising n type AlGaInP, a first p type cladding layer 38comprising p type GaAs, an active layer 39 comprising GaAsP, an n typecurrent blocking layer 40 comprising n type GaAs, a second p typecladding layer 41 and a contact layer 42 both comprising p type GaAs,and an electrode 10. This BR laser is fabricated by successively growingon an n type GaAs semiconductor substrate 36 an n type AIGaInP claddinglayer 37, a GaAsP active layer 39, and a first p type GaAs claddinglayer 38 by MOCVD, forming a stripe shaped mask (not shown) comprisingan insulating film, etching to the first p type GaAs cladding layer 38to a depth not reaching the GaAsP active layer 39 thereby to form aridge, regrowing the n type GaAs current blocking layer 40 by MOCVD tobury the ridge employing the mask as a selective growth mask, and, afterremoving the mask, successively regrowing the second p type claddinglayer 41 and contact layer 42, both comprising p type GaAs employingMOCVD. During this process, laser deterioration occurs at the regrown pnjunction interface 43 between the first p type cladding layer 38 and then type current blocking layer 40 and of the crystalline regrowthinterface 44 between the second p type cladding layer 41 and the n typecurrent blocking layer 40.

As discussed above, in the prior art BH, BC, and BR lasers, there areimpurities and natural oxides films at regrown pn junction interfaces.Accordingly, as the operation current increases, the idle currentincreases. With this idle current flowing in a high temperaturecondition, the deterioration of regrown pn junction interfaces isaccelerated, thereby causing a change in the operation current,resulting in laser deterioration, which in turn results in deterioratedreliability.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductorlaser that solves the problem described above and that has a highreliability.

Other objects and advantages of the present invention will becomeapparent from the detailed description given hereinafter; it should beunderstood, however, that the detailed description and specificembodiment are given by way of illustration only, since various changesand modifications within the scope of the invention will become apparentto those skilled in the art from this detailed description.

According to the present invention, a semiconductor laser includes afirst conductivity type cladding layer on a first conductivity typesemiconductor substrate; an active layer for laser oscillation on thefirst conductivity type cladding layer; a first second conductivity typecladding on the active layer, the second conductivity type beingopposite to the first conductivity type, current blocking layers at bothsides of the active layer to converge current flow in the active layer;a second second conductivity type cladding layer in contact with theupper surface of the current blocking layer, wherein the interfacebetween the first conductivity type cladding layer and first secondconductivity type blocking layer and the current blocking layers and theinterface between the second second conductivity type cladding layer andthe current blocking layers are crystalline regrowth interfaces; and adoped layer including at least one of Fe, Cr, and Co atoms at thecrystalline regrowth interface. At least one of Fe, Cr, and Co entersthe crystaline lattices so the movement of impurities at the crystallineregrowth interfaces is suppressed, thereby suppressing laserdeterioration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a semiconductor laseraccording to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view illustrating a semiconductor laseraccording to a second embodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating a semiconductor laseraccording to a third embodiment of the present invention.

FIG. 4 is a cross-sectional view illustrating a semiconductor laseraccording to a fourth embodiment of the present invention.

FIGS. 5(a) and 5(b) are diagrams showing the results of measurements ofthe change in operational current at an output of 5 mW after operationfor 3000 hours of the prior art device and a device according to thepresent invention, respectively.

FIG. 6 is a cross-sectional view illustrating a structure of a prior artBH laser having a semiconductor substrate comprising p type InP.

FIGS. 7(a)-7(d) are diagrams illustrating a method of fabricating aprior art BH laser having a semiconductor substrate comprising p typeInP.

FIG. 8 is a cross-sectional view illustrating a structure of a prior artBH laser having a semiconductor substrate comprising p type InP.

FIG. 9 is a cross-sectional view illustrating a structure of a prior artBH laser having a semiconductor substrate comprising n type InP.

FIG. 10 is a cross-sectional view illustrating a structure of a priorart BR laser having a semiconductor substrate comprising n type GaAs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments 1 and 2

FIG. 1 is a cross-sectional view illustrating a semiconductor laseraccording to a first embodiment of the present invention. In the figure,reference numerals 1-14 designate the same elements as in FIG. 6 whichshows a BH laser employing a semiconductor substrate comprising p typeInP.

The structure of FIG. 1 includes a layer 50 containing Fe in thevicinity of the regrown pn junction interfaces 11 and 12 between thefirst n type cladding layer 5 and the p type current blocking layer 6and at the crystalline regrowth interfaces 13 and 14 between the secondn type cladding layer 8 and the p type current blocking layer 6.

The semiconductor laser of this first embodiment is formed using amethod similar to that for the prior art semiconductor laser shown inFIG. 6. The doped layer 50 is formed by including a source of Fe duringthe MOCVD step immediately before growing the p type current blockinglayer 6 and the n type current blocking layer 7, after growing the ptype current blocking layer 6 and the n type current blocking layer 7,and immediately before growing the second n type cladding layer 8. Asthe Fe dopant concentration, 1×10¹⁶ -1×10¹⁷ cm⁻³ is preferable.

FIGS. 5(a) and 5(b) respectively show changes in the operational currentrelative to the initial operation current for lasers according to theprior art and the present invention operated at a temperature of 50° C.and at a constant current for 3000 hours. Thereafter, the lasers wereoperated at 5 mW output, with applied current as a parameter.

As is apparent from the figures, there is a tendency for the current toincrease when the operational current is high in the prior art structureof FIG. 5(a), while in the structure of FIG. 5(b), having the dopedlayer 50 including Fe, the operational current does not change even witha high operational current, whereby a high reliability is obtained.

The deterioration of the pn junction interface is considered to becaused by a change of crystalline arrangement at the regrowth interfacesand movement (diffusion) of impurity atoms therein. The change ofcrystalline arrangement and the movement of impurity atoms are promotedbecause atoms move between crystalline lattices. Generally, because Featoms enter into a crystalline lattice of InP without occupying In or Plattice sites, they suppress changes in crystalline arrangement and themovements of impurity atoms. Therefore, in this embodiment, changes incrystalline lattices at the regrowth interfaces and the movement ofimpurities are suppressed by the Fe atoms, whereby the deterioration ofthe semiconductor laser can be suppressed.

Here, the doped layer 5 at the interface between the first n typecladding layer 5 and the second n type cladding layer 8 has aconcentration of Fe atoms that is not high enough to make the dopedlayer 50 have a high resistance, i.e., a concentration of 1×10¹⁶ -1×10¹⁷cm⁻³, whereby there are no unfavorable influences on the lasercharacteristics.

The structure described above may be fabricated using liquid phaseepitaxy as in the prior art semiconductor laser shown in FIG. 8. Duringthat epitaxy, by using a melt including Fe in contact with thecrystalline regrowth interfaces 24 and 25 immediately before growing thep type cladding layer 19, an Fe-doped layer 50 is formed.

BC lasers having the conventional structure and this embodiment,respectively, were operated at a constant current at a temperature of50° C. for 3000 hours and, thereafter, the change in the operationalcurrent at 5 mW output relative to the initial operational current wasmeasured with the applied current as a parameter. The results show thatwhen the operational current was high in the prior art structure, thecurrent increases, while the operational current does not change even ata high applied current in a structure according to this embodiment.Thus, high reliability is obtained as in FIGS. 5(a) and 5(b).

While in the first and second embodiments, p type InP is used for thesemiconductor substrate 1, an n type InP substrate may be employed.

Embodiment 3

FIG. 3 is a cross-sectional view illustrating a semiconductor laseraccording to a third embodiment of the present invention. In the figure,reference numerals 10 and 26-35 designate the same elements as in theprior art BH laser shown in FIG. 9 that employs an n type InPsemiconductor substrate.

This semiconductor laser has doped layers 50 containing Fe atomsdeposited during MOCVD growth at the regrown pn junction interface 34between the n type cladding layer 28 and the p type current blockinglayer 30 and the crystalline regrowth interface 35 between the first ptype cladding layer 29, the second p type cladding layer 32, and the ntype current blocking layer 31. This laser is fabricated by the samemethod as that for the prior art BH laser shown in FIG. 9. An Feconcentration of 1×10¹⁶ -1×10¹⁷ cm⁻³ is preferable.

The BH lasers having the structures of this third embodiment and theprior art were operated at 50° C. and a constant current for 3000 hoursand, thereafter, the change in the operational current at 5 mW outputwas compared to the initial operation current with the applied currentas a parameter. Although there is a tendency in the prior art structurethat the operational current increases when the operational current ishigh, as in the FIGS. 5(a) and 5(b), the operational current does notchange even at a high current in a structure according to this thirdembodiment having the doped layer 50 containing Fe, thereby achievinghigh reliability.

While in the third embodiment n type InP is used for the semiconductorsubstrate 26, p type InP may be employed therefor.

Embodiment 4

FIG. 4 is a cross-sectional view illustrating a semiconductor laseraccording to a fourth embodiment of the present invention. In the figurereference numerals 10, 36-44 designate the same elements as those in theprior art BH laser shown in FIG. 10 that employs an n type GaAssemiconductor substrate.

As shown in the FIG., the laser structure according to this fourthembodiment includes doped layers 50 containing Fe incorporated duringMOCVD growth at the regrown pn junction interfaces of the layer 43 andthe first p type cladding layer 38 and the n type current blocking layer40, and the layer 44 and the second p type cladding layer 41 and the ntype current blocking layer 40. This semiconductor laser is fabricatedby a method similar to that for the prior art semiconductor laser shownin FIG. 10, and in which a doped layer 50 is formed at the crystallineregrowth interface, containing Fe atoms, immediately before the growthof the n type current blocking layer 40 and the second p type claddinglayer. As dopant concentration Fe atoms, 1×10¹⁶ -1×10¹⁷ cm⁻ ispreferable.

The BR lasers of this embodiment and the prior art structures wererespectively operated at 50° C. and a constant current for 3000 hoursand, thereafter, the change in the operational current at an output of 5mW relative to the initial operational current was measured with theapplied current as a parameter. Similar to FIGS. 5(a) and 5(b), althoughthere is a tendency that current increases when the operational currentis high in the prior art structure, the operational current does notchange in a structure according to this fifth embodiment provided withthe doped layer 50 including Fe, whereby high reliability is obtained.

While in the fourth embodiment GaAs is used as the semiconductorsubstrate, InP may be employed as the substrate.

While in the first to fourth embodiments, the doped layer 50 containsFe, the doped layer 50 may include Cr or Co. In addition, the dopedlayer 50 may be doped with more than two dopants, e.g., Fe, Cr, and Co,with the same effects as in the first to fourth embodiments

While a BH laser, a BC laser, and a BR laser are described, the presentinvention may be applied to semiconductor lasers which are fabricatedemploying other crystalline regrowth processes with the same effects.

What is claimed is:
 1. A semiconductor laser comprising:a firstconductivity type semiconductor substrate; a first conductivity typelower cladding layer disposed on said substrate; an active layer forlaser oscillation disposed on said first conductivity type lowercladding layer; a second conductivity type first upper cladding layerdisposed on said active layer, the second conductivity type beingopposite to the first conductivity type; current blocking layersdisposed at opposite sides of said active layer for confining currentflow to said active layer; a second conductivity type second uppercladding layer in contact with an upper surface of said current blockinglayers wherein interfaces between said current blocking layers and (i)said lower cladding layer, (ii) said first upper cladding layer, and(iii) said second upper cladding layer are regrowth interfaces; anddoped layers including at least one of Fe, Cr, and Co atoms as a dopantdisposed at said regrowth interfaces.
 2. The semiconductor laser ofclaim 1, wherein said semiconductor substrate comprises InP.
 3. Thesemiconductor laser of claim 1, wherein said lower cladding layer, saidactive layer, and said first upper cladding layer have a mesa shape andsaid current blocking layers bury said mesa shape.
 4. The semiconductorlaser of claim 3, wherein said semiconductor substrate comprises InP. 5.The semiconductor laser of claim 1, wherein said semiconductor laser hasa buried crescent laser structure and said lower cladding layer, saidactive layer, and said first upper cladding layer are disposed in agroove in said current blocking layers, the groove reaching saidsemiconductor substrate.
 6. The semiconductor laser of claim 5, whereinsaid semiconductor substrate comprises InP.
 7. A semiconductor lasercomprising:a first conductivity type semiconductor substrate; a firstconductivity type lower cladding layer disposed on said substrate; anactive layer for laser oscillation disposed on said first conductivitytype lower cladding layer; a second conductivity type first uppercladding layer disposed on said active layer, the second conductivitytype being opposite to the first conductivity type; current blockinglayers disposed at opposite sides of said active layer for confiningcurrent flow to said active layer; a second conductivity type secondupper cladding layer in contact with an upper surface of said currentblocking layers wherein interfaces between said current blocking layersand (i) said lower cladding layer and (ii) said second upper claddinglayer are regrowth interfaces; and doped layers including at least oneof Fe, Cr, and Co atoms as a dopant disposed at said regrowthinterfaces.
 8. The semiconductor laser of claim 7, wherein saidsemiconductor substrate is selected from the group consisting of GaAsand InP;said first upper cladding layer is a ridge; and said currentblocking layers bury said ridge.
 9. A semiconductor laser comprising:afirst conductivity type semiconductor substrate; a mesa-shaped stripestructure comprising a first conductivity type lower cladding layerdisposed on said first conductivity type semiconductor substrate, anactive layer for laser oscillation disposed on said lower claddinglayer; and a second conductivity type first upper cladding layerdisposed on said active layer; current blocking layers burying bothsides of the mesa-shaped stripe structure; a second conductivity typesecond upper cladding layer disposed on upper surfaces of said currentblocking layers and said first upper cladding layer; and doped layersincluding at least one of Fe, Cr, and Co atoms as a dopant disposed atinterfaces of said current blocking layers and (i) said second uppercladding layer, (ii) said active layer, (iii) said first upper claddinglayer, and (iv) said second upper cladding layer.
 10. The semiconductorlaser of claim 9, wherein said semiconductor substrate comprises InP.11. The semiconductor laser of claim 10, wherein said doped layercontains Fe atoms in a concentration of 1×10¹⁶ ˜1×10¹⁷ cm⁻³.
 12. Asemiconductor laser comprising:a first conductivity type semiconductorsubstrate; current blocking layers disposed on said semiconductorsubstrate and having a stripe-shaped groove having a depth reaching saidsemiconductor substrate; a first conductivity type lower cladding layer,an active layer, and a second conductivity type first upper claddinglayer successively disposed in and filling the stripe-shaped groove insaid current blocking layers, and a second conductivity type secondupper cladding layer disposed on said first upper cladding layer andsaid current blocking layers wherein interfaces between said currentblocking layers and (i) said lower cladding layer, (ii) said first uppercladding layer, and (iii) said second upper cladding layer are regrowthinterfaces; and doped layers including at least one of Fe, Cr, and Coatoms as a dopant disposed at said regrowth interfaces.
 13. Thesemiconductor laser of claim 12, wherein said semiconductor substratecomprises InP.
 14. The semiconductor laser of claim 12, wherein saiddoped layer contains Fe atoms in a concentration of 1×10¹⁶ ˜1×10¹⁷ cm⁻³.15. A semiconductor laser comprising:a first conductivity typesemiconductor substrate; a first conductivity type lower cladding layerdisposed on said substrate; an active layer disposed on said lowercladding layer; a second conductivity type first upper cladding layerdisposed on said active layer and including a ridge; current blockinglayers burying both sides of said ridge; a second conductivity typesecond upper cladding layer disposed on said current blocking layers andsaid first upper cladding layer; and doped layers including at least oneof Fe, Cr, and Co atoms as a dopant disposed at an interface of saidcurrent blocking layers and said first upper cladding layer and aninterface of said current blocking layers and said second upper claddinglayer.
 16. The semiconductor laser of claim 15, wherein said firstconductivity type semiconductor substrate comprises one of GaAs and InP.17. The semiconductor laser of claim 16, wherein said doped layercontains Fe atoms in a concentration of 1×10¹⁶ ˜1×10¹⁷ cm⁻³.